This invention relates to a method of manufacturing and a die therefor by which components such as pistons may be forged.
In the interest of reducing weight and improving performance, internal combustion engines normally utilize pistons formed from aluminum or aluminum alloy. Conventionally, these pistons are formed by a casting process. However, when utilizing casting, the full advantages of the aluminum material are not realized. Also, because of the casting process it is generally necessary to make the piston somewhat larger and heavier in order to provide adequate strength, wear resistance and durability.
The full advantages of the aluminum material can be utilized better if the pistons are made by a forging process. However, in connection with the forging of pistons, certain problems may arise.
That is, it is desirable to provide the piston with a dense and relatively uniform structure. However, due to the shape of the piston, this is difficult with conventional forging die, particularly if the forging is done in a single step. That is, the lower portion of the piston, specifically, the lower edges of the skirt and the ribs that connect the skirt to the piston pin bosses require rather extensive extrusion of the material in the forging die.
The area where these parts of the piston are extruded into generally is narrow and there is a problem that cracking of the die in this area may result. In addition, because of the amount of extrusion necessary to form these components of the piston, there is also some possibility of porosity being encountered at the lower extremities of these components.
These problems may be best understood by reference to FIGS. 1-3, which show a forged piston of a configuration of the type with which the invention has particular utility and FIGS. 4 and 5 which are cross-sectional views taken through a conventional forging die arrangement showing the blank in place in FIG. 4 and the finished forging step in FIG. 5.
As seen in FIGS. 1-3, a conventional forged piston is indicated generally by the reference numeral 21. This forged piston 21 is formed from aluminum alloy and specific materials which may be advantageously used may be described later in the specification. The piston is comprised of a head portion, indicated generally by the reference numeral 22 and which is comprised primarily of an upper surface 23 which cooperates with a recess formed in the cylinder head and the cylinder bore to form the combustion chambers of the engine.
Below this head surface 23, the head portion 22 is formed with one or more ring grooves 24 in which piston rings are provided for effecting sealing with the associated cylinder bore, These are normally formed by machining the forged blank at the completion of the forging process.
Depending from the head portion 22 on opposite sides thereof are a pair of generally spaced apart skirt portions 25. These skirt portions 25 are disposed on opposite sides of boss portions 26 in which piston pin receiving openings 27 are formed. Again, the pin receiving openings are machined at the completion of the forging operation. The boss portions 26 are interconnected to the skirt portions 25 by ribs 28.
Thus, it will be seen that the lower portions of the skirt portions 25 and the ribs 28 must undergo significant extrusion in the forging process in order to form the piston. This may be understood by reference to FIGS. 4 and 5 which show the forging apparatus by which the piston 21 is formed.
As seen in these figures, there is a female die indicated generally by the reference numeral 29 and which is comprised primarily of two parts consisting of a lower end closure 31 and a cylindrical body member 32. The end closure 31 has projecting portions that extend upwardly into a cylindrical cavity 33 formed by the cylindrical portion 32. This cavity is comprised of a pair of side parts 34 which have a deep extent and in which the skirt portions 25 are formed. In addition, there are a pair of circumferentially spaced recessed parts 35 that form the rib portions 28. These also require substantial extrusion of the material.
A forging male die 36 completes the die assembly and cooperates with the cavities 33, 34, and 35 to form the piston 21.
As may be seen, a blank 37 of the piston material is inserted into the cavity 33 at the upper portion thereof. The pressing die 36 then moves downwardly so as to force the blank into the cavity and downwardly into the portions 34 and 35 so as to form the finished blank, indicated by the reference numeral 38.
As may be seen, the deep extrusion of the skirt portion 25 and to some extent the rib portions 28 causes two significant problems. First, the clearance in the die is very small in these areas and hence there is a large expansion force that tends to cause cracking the die. In addition, air pockets may form in the lower part of these areas and cause porosity or irregularities in the lower shapes of the skirts 25 and ribs 28.
It is, therefore, a principal object of this invention to provide an improved forging die and method of forging a piston that will avoid these problems.
It is a further object of this invention to provide a forging die that permits the formation of the skirt and rib portions of the piston without the likelihood of cracking of the die and without causing porosity to exist in these portions of the finished piston.